Text (UD+errata): Each other non-Aura enchantment is a creature with power and toughness each equal to its converted mana cost. It's still an enchantment. [Oracle 2005/08/01]
An enchantment that turns into a creature cannot attack unless it started your most recent turn in play, and you have controlled it continuously since that time. [Urza's Destiny FAQ 1999/05/25] See Rule 302.6.
If one Opalescence and Humility are in play, then Opalescence turns Humility into a 4/4 creature, then Humility turns itself into a 1/1 creature with no abilities. Yes, this is counter-intuitive since Humility no longer has the ability to remove abilities, but this is the outcome. The timing rules for the interaction of continuous effects say you apply them in this order and you never loop back to see if that application would change things. [D'Angelo 1999/05/01] If new creatures or enchantments come into play, you apply Opalescence and Humility in order, so they come into play as 1/1 with no abilities. [D'Angelo 1999/07/27]
If Humility is in play with more than one Opalescence, then you apply the Opalescence effects in order, then apply Humility last. So all enchantments (including all of the Opalescences) end up being 1/1 creatures with no abilities. [WotC Rules Team 2003/09/01]
Does not animate itself. But can be animated by another Opalescence. [DeLaney 1999/06/08]
